Various heat pumps are known which operate in accordance with the compression or absorption principle. In these heat pumps, readily vaporizable liquids having a low vapor pressure such as halohydrocarbons or ammonia are compressed mechanically or thermally until liquefaction begins, and the condensation heat of the particular working materials is obtained as heating energy or available heat. The available heat consists of the enthalpy of vaporization which is contributed by environmental energy and the compression heat originating from the mechanical or thermal drive. Thus, merely changes of the state of aggregation take place and chemical changes are avoided intentionally.
In compression heat pumps which are operated electrically, the performance numbers, i.e. the ratio of delivered available heat to expended auxiliary energy, range between 2 and 4. In absorption type heat pumps which are basically operated with fossil energy, this number is about 1.3. As compared herewith, an oil or gas heating boiler has a performance number of about 0.8.
Due to the general energy shortage, interest was recently attracted also by thermochemical heat pumps where utilization of the absorption or output of energy in a reversible chemical reaction is tried. It is an advantage of thermochemical heat pumps over the previously used heat pumps that, for maintaining the enthalpy of a chemical reaction, far lower amounts of auxiliary energy are generally needed than for pure compression and/or condensation processes. This means theoretically that thermochemical heat pumps should be capable of higher performance numbers than the known heat pumps operating on a pure physical basis. Heretofore, especially the alkaline earth metal chloride hydrates or ammoniacates have been investigated as reversible chemical reactions. These systems appeared to be interesting especially in connection with the storage of heat such as, for example, solar energy; see DE-OS No. 27 58 727 and DE-OS No. 28 10 360. These systems attained substantially no importance so far since various requirements must be met which are not or only incompletely complied with by these chemical systems:
(1) Full reversibility of the chemical reaction, which is equivalent to long cycle lifetime of the working materials.
(2) As high a reaction enthalpy as is possible associated with the additional requirement that the energy-absorbing process takes place at as low a temperature as is possible (utilization of environmental energy of low energy level) and the energy-yielding process furnishes thermal energy on a temperature level which is sufficient to be capable of operating at least heating installations of buildings.
(3) The course with respect to reaction kinetics must fully satisfy the demands made, i.e. the system must not operate too slowly.
(4) Satisfactory thermal conductivity of the working materials to minimize impediment of the heat exchange process.
(5) Freedom from toxicity of the working materials in order that no health hazards are caused in case of any leakage of the normally fully encapsulated heat pump system.
(6) Reasonable and justifiable price of the working materials.
At temperatures below the freezing point, the rate of dissociation and vaporization of alkaline earth metal chloride hydrates is not high enough. Therefore, they can be operated only with the aid of heat from the ground, from flowing bodies of water or groundwater, which restricts the field of application considerably. In any case, the ambient air which is available to everybody cannot be used as an energy carrier at temperatures below the freezing point.
Moreover, the thermal conductivity of the previously proposed working materials is low so that considerable problems are encountered in the heat exchange processes. At least very large heat exchange surfaces are necessary in case of the previously proposed working materials, which results in units which have an undesirably great volume.
Further substantial difficulties result from mass and energy transport. Thus, the rate of the reaction is decreased to the extent to which anhydrous or ammonia-free salts become coated with layers of salt hydrate or ammoniacate. Distribution of the working materials over a large surface area is unavoidable also for this reason.
In recent years, some metal hydrides have been subjected to closer investigations with a view to use them perhaps for the recovery and storage of hydrogen which can be considered on principle as alternative energy for both engines and heating installations. The hydride formation or hydride cleavage involves a substantial change of enthalpy, which results in considerable difficulties and disadvantages in the case of the intended uses of these metal hydrides. Therefore, the proposal was already made for test vehicles to use the waste heat of the motor and exhaust gases for heating the hydride reservoir. In the summer months, direct air conditioning is possible by heat exchange with the hydride reservoir. On the other hand, great difficulties are encountered in the starting phase because a sufficient hydrogen pressure must be present even at low temperatures to start the motor and bridge over the period of time until the exhaust gases are sufficiently warm to be used for heating the hydride reservoir. Therefore, a combined hydrogen storage system has already been proposed in which tanking-up of the vehicle and heating of the building are combined and the liberated amounts of energy of hydride formation are utilized advantageously; see H. Buchner, Das Wasserstoff-Hydrid-Energiekonzept, Chemie Technik 7 (1978), pp. 371-377. Accordingly, about 30% of the heat content of hydrogen at room temperature can be converted into available heat of higher temperature by hydride formation. Therefore, the recommendation is given to couple always the hydrogen recovery and heat recovery in this process.
As a reversal of this concept, the proposal was also made to store solar heat for air conditioning of buildings by means of metal hydrides. The primary energy source is assumed to be a flat solar collector of about 100.degree. C. and the auxiliary heat bath is assumed to be the ground on a temperature level of about 10.degree. C. As heat accumulator and heat transformation, there are provided two metal hydride reservoirs which contain CaNi.sub.5 and Fe.sub.0.5 Ti.sub.0.5 powder and between which hydrogen gas can be exchanged by opening a valve. Moreover, heat exchangers connect the two hydride reservoirs with the primary energy source, with the auxiliary heat bath or with the consumer, a building; see H. Wenzl, Wasserstoff in Metallen: Herausragende Eigenschaften and Beispiele fur deren Nutzung, Kernforschungsanlage Juelich GmbH, January, 1980, pp. 66, 67 and FIG. 13. However, a rough estimate shows that this concept has not a chance of being realized because it would be necessary to use hydride reservoirs with dimensions which are much too large to be able to serve as storage of solar energy in profitable dimensions.